1. Field of the Invention
The present invention relates to a wide range azimuth (AZ) driving system for a satellite communication antenna which is set at an earth station of a satellite communication system, and more particularly, to a wide range azimuth (AZ) driving system for an antenna in view of driving at a low speed for tracking of a geostationary satellite and driving at a high speed for re-positioning.
2. Description of the Related Art
Hitherto, the wide range azimuth (AZ) driving system for the antenna is used as a wide range azimuth (AZ) driving system for a satellite communication antenna which is set at an earth station of a satellite communication system using, for example, a geostationary satellite shown in FIG. 1.
Systems such as INTELSAT (Internal Telecommunications Satellite Consortium) and INMARSAT (International Maritime Satellite Organization) are well known as satellite communication systems to which the antenna is applied.
As shown in FIG. 1, a wide range azimuth (AZ) driving device 13 for an antenna as the above-mentioned system is disposed at a base portion of a yoke for supporting an antenna 9 and drives the antenna 9 in the azimuth (AZ) direction.
As an example of the outline of a size in a system which is put into practical use, an aperture diameter D1 of the antenna 9 is within a range of approximately 3.6 (m) as one of a small size to 18 (m) as one of a maximum size and a main body of the antenna 9 is approximately 16 (t) in weight at the maximum. A diameter D2 of a large gear 10 is approximately 1.8 (m).
As one example of the size in antennas which are widely used, the aperture diameter D1 of the antenna 9 is approximately 10 (m), the main body of the antenna 9 is approximately 4 (t) in weight, and the diameter D2 of the large gear 10 is approximately 1.2 (m).
The antenna in the satellite communication system is driven in the azimuth (AZ) direction by the following two operations.
According to a first operation, an antenna is azimuthally (AZ) driven by switching geostationary satellites which communicate data. The geostationary satellite communication system comprises a plurality of geostationary satellites and data which is communicated every satellite is different. As shown in FIG. 1, the antenna 9 which communicates data with an A geostationary satellite 14-1 is azimuthally (AZ) driven in a direction (shown by reference numeral 9' in FIG. 1) of a B geostationary satellite 14-2 so as to communicate another data. The operation is called as wide range driving.
The wide range driving requires that a target satellite is acquired as quickly as possible. Therefore, the antenna 9 also requires driving at a high speed throughout a wide range.
According to a second operation, an antenna is azimuthally (AZ) driven so as to track a satellite which the antenna acquires. The position of the geostationary satellite which is seen from the earth always changes by the deviation from an orbit. FIG. 2 shows a state of the change in the position of the satellite which is seen from the earth. This is called as 8-shaped movement of the satellite, the satellite draws an 8-shaped locus on one-day cycle while the position at which the satellite should inherently remain stationary is central. As one example, an azimuth (AZ) tracking range (azimuth range) of the 8-shaped movement is approximately 6.degree. in the azimuth (AZ) direction at the maximum.
However, a range of directional characteristics (azimuth range) of the aforementioned antennas which are generally used is 0.024.degree., that is, remarkably narrow, as one example of the systems which are put into practical use.
Therefore, if the antenna acquires the satellite once, the satellite deviates from the orbit by the 8-shaped movement and, thus, deviates from the range of the directional characteristics of the antenna and cannot maintain a predetermined antenna gain continuously.
Then, the antenna is driven in the azimuth (AZ) direction in accordance with the 8-shaped movement of the satellite and tracks the satellite. The operation is called as tracking driving.
The tracking driving requires that the azimuth of the antenna is accurately controlled. Accordingly, it is necessary to suppress backlash of a gear of a driving system.
Incidentally, referring to FIG. 2, the 8-shaped movement of the satellite includes not only the displacement of the azimuth (AZ) direction but also the displacement in the elevation (EL) direction. However, the present invention includes no driving of tracking in the elevation (EL) direction and the description is omitted.
FIG. 3 is a structural diagram illustrating one example of wide range azimuth (AZ) driving systems for an antenna according to a conventional technique which is used for the operations.
Referring to FIG. 3, the wide range azimuth (AZ) driving system for the antenna according to the conventional technique comprises one large gear 10, an A small gear 1 and a B small gear 3 for driving the large gear 10, two gear reducers of an A small gear 15-1 and a B small gear 15-2, two motors of an A motor 16-1 and a B motor 16-2, two clutches of an A clutch 5-1 and a B clutch 5-2 for connecting/disconnecting the A gear reducer 15-1 and the B gear reducer 15-2 to the A motor 16-1 and the B motor 16-2, an antenna control unit (ACU) 19, a torque bias adding circuit (TB) 17, and two servo amplifies of an A servo amplifier 18-1 and a B servo amplifier 18-2.
The ACU 19 is instructed on the azimuth to which the antenna 9 is directed from the outside, and instructs the A servo amplifier 18-1 and the B servo amplifier 18-2 on the rotational direction, rotational speed, and stop of the A motor 16-1 and the B motor 16-2. The ACU 19 also detects the azimuth (AZ) direction to which the antenna 9 is directed. As one example of the systems which are put into practical use, the ACU 19 has performance for detecting the azimuth (AZ) direction with precision of a level ranging from 1/100.degree. to 1/1000.degree.. Specifically speaking, the azimuth (AZ) direction is determined every system in accordance with the range of the directional characteristics of the antenna (azimuth range).
The TB 17 measures consumption currents of the two motors of the A motor 16-land the B motor 16-2, thereby detecting loads of the A motor 16-1 and the B motor 16-2 and controlling the A servo amplifier 18-1 and the B servo amplifier 18-2 so as to cause torque biases in the two motors.
The torque bias will be simply described. According to the structure shown in FIG. 3, the A motor 16-1 and the B motor 16-2 suppress backlash which is caused among the A small gear 1, the B small gear 3, and the large gear 10 by causing a torque bias when the azimuth of the antenna is maintained and the tracking by the antenna 9 is driven.
When the azimuth (AZ) direction of the antenna 9 is maintained, the ACU 19 controls the A servo amplifier 18-1 and the B servo amplifier 18-2 so as to stop the A motor 16-1 and the B motor 16-2. The TB 17 controls the A servo amplifiers 18-1 and the B servo amplifier 18-2 so that the A motor 16-1 and the B motor 16-2 generate driving forces having a predetermined intensities which are mutually directed in the opposite direction. Thus, the A servo amplifier 18-1 drives the A motor 16-1 that, for instance, the A motor 16-1 rotates in the right direction and the B servo amplifier 18-2 drives the B motor 16-2 that, for instance, the B motor 16-2 rotates in the left direction. The TB 17 detects the loads of the motors and controls the motors so that the mutual driving forces are balanced. Accordingly, the backlash is suppressed and the azimuth of the antenna 9 is precisely maintained. The driving force is called as a torque bias.
Incidentally, the A clutch 5-1 and the B clutch 5-2 are provided for purpose of disconnecting a troubled motor to the reducers, mainly when the motor is troubled. Therefore, during the normal operation, the operation for controlling that the motors are connected to the reducers and, consequently, the detailed description is omitted.
Next, the operation of FIG. 3 will be described with reference to FIGS. 4a to 4c.
FIGS. 4a to 4c illustrate the control sequence of the ACU 19 in the FIG. 3.
Referring to FIG. 4a, when the system is started, the ACU 19 connects the A clutch 5-1 and the B clutch 5-2 to the A motor 16-1 and the B motor 16-2 (step S61), the A motor 16-1 and the B motor 16-2 are stopped (step S62), and the TB 17 is controlled and a torque bias is added (step S63).
Thus, the A motor 16-1 and the B motor 16-2 are connected to the A gear reducer 15-1 and the B gear reducer 15-2, respectively. The A motor 16-1 and the B motor 16-2 generate torque biases. The backlash between the A small gear 1 and B small gear 3 and the large gear 10 is suppressed and the antenna becomes stationary. This results in assuring the initial state.
Next, the operation of the wide range driving for acquiring the satellite will be described.
When the direction to which the antenna is directed is instructed, the ACU 19 detects the current direction to which the antenna 9 is directed and calculates necessary rotational direction and rotational angle. If the rotational angle is larger than a certain extent thereof, the wide range driving is controlled.
Referring to FIG. 4b, the torque bias is first reset when the wide range driving (step S71). Because no communication is performed during the wide range driving, the direction to which the antenna 9 is directed needs no precision. Sequentially, the A motor 16-1 and the B motor 16-2 are driven in the designated direction at a high speed (step S72). The direction to which the antenna 9 is directed is detected (step S73). When the antenna 9 is directed in the designated direction, the A motor 16-1 and the B motor 16-2 are stopped (step S74). The torque bias is added (step S75), and the wide range driving ends.
Obviously, in case of the actual azimuth (AZ) driving for the antenna, it is necessary to properly control the acceleration and the deceleration of the A motor 16-1 and the B motor 16-2 when the wide range driving is started and stopped so as to properly suppress the moment of inertia which derives from the weight of the antenna and to correctly assure a target direction. However, this is a well-known technique and the detailed description is herein omitted.
Subsequently, the description is given to the operation of tracking driving for tracking the satellite.
When the direction to which the antenna 9 should be directed is instructed, the ACU 19 detects the direction to which the antenna 9 is currently directed and calculates the rotational direction and the rotational angle. If the rotational angle is larger than a certain extent thereof, the tracking driving is controlled.
Referring to FIG. 4c, the A motor 16-1 and the B motor 16-2 are driven at a predetermined low speed during the tracking driving (step S81), and the direction to which the antenna 9 is directed is detected (step S82). When the antenna 9 is directed in the designated direction, the A motor 16-1 and the B motor 16-2 are stopped (step S83).
The communication through the antenna 9 is continued during the tracking driving, so that the torque bias is continuously added because of driving with high precision.
The detailed description is given to the operation when the A servo amplifier 18-1 and the B servo amplifier 18-2 are simultaneously controlled by both the ACU 19 and the TB 17 in the case in which the ACU 19 is instructed on the direction to which the antenna 9 is directed and drives the tracking of the antenna 9.
Before start of the tracking driving, the large gear 10 becomes stationary in a state in which the backlash is absent and the A motor 16-1 and B motor 16-2 mutually adds a torque bias. It is assumed that the A small gear 1, the B small gear 3, the A motor 16-1, and the B motor 16-2 rotate in the left direction by the tracking driving, and the A motor 16-1 and the B motor 16-2 are DC motors, and drive voltages of the motors are positive when rotating in the left direction. It is also assumed that A servo amplifier 18-1 outputs a drive voltage -VT (V) and the B servo amplifier 18-2 outputs a drive voltage +VT (V) due to the addition of the torque bias.
When the start of the tracking operation, the ACU 19 rotates both the A servo amplifier 18-1 and the B servo amplifier 18-2 in the left direction, instructs the rotational speed corresponding to the calculated rotational angle, and controls the driving of the A motor 16-1 and the B motor 16-2. It is assumed that under the control operation by the ACU 19, the drive voltages which are inputted to the A servo amplifier 18-1 and the B servo amplifier 18-2 are VL (V).
The A servo amplifier 18-1 and the B servo amplifier 18-2 add the torque bias by the TB 17 to the motor drive voltage by the ACU 19 and output a drive voltage VD.
Therefore, when an output drive voltage of the A-servo amplifier 18-1 is VD1, EQU VD1=VL-VT(V) (1)
Similarly, an output drive voltage of the B servo amplifier 18-2 is VD2, EQU VD2=VL+VT (V) (2)
According to the structure of FIG. 3, the antenna is driven in the azimuth (AZ) direction while the torque bias is imparted during the tracking driving and the backlash of the large gear 10 is suppressed.
As mentioned above, the conventional geostationary satellite communication system acquires and tracks any desired geostationary satellite by using the azimuth (AZ) driving system for the antenna and communicates data.
The foregoing wide range azimuth (AZ) driving system for the antenna according to the conventional technique comprises the two motors of the A motor 16-1 and the B motor 16-2 and the two-system driving-mechanism corresponding thereto and, therefore, the structure is complicated. Because opposite-polarized torque biases are imparted to the two motors and the backlash of the two driving systems are controlled to be suppressed.
The opposite-polarized torque biases are imparted to the two motors and the backlash of the two driving systems is suppressed and, therefore, the TB 17, the A servo amplifier 18-1, and the B servo amplifier 18-2 are necessary and the structure becomes complicated. This causes a serious problem in the case in which costs are reduced.
Further, since the performances of the motors are individually varied every motor, the relationship between the load (consumption current value) of the motor which is detected by the TB 17 and the driving force which is generated by the motor is not uniform, depending on the system. The imparting amount of torque bias has to be adjusted every system in view of the balance of driving forces of the two motors. This work needs a high-level technique and a long time and causes a serious problem in the case of completing the system for a short time.